Polymeric electrolyte membrane fuel cells (PEMFC) are being developed for gaseous hydrogen fuel feed at the negative electrode (anode) for producing water at the positive electrode (cathode) by combining hydrogen ions migrating through the polymeric electrolyte with the oxygen of an air stream supplied to the cathode. The use of less hazardous and naturally available methanol in aqueous solution as fuel has advantages especially in terms of a safer portability and availability of fuel.
The overall reaction occurring in a direct methanol fuel cell (DMFC) is the same as that of direct combustion of methanol:CH3OH+3/202→CO2+2H2O
Considering that a fuel cell operates isothermally, all the free energy associated with the above reaction should in principle be convertible to electrical energy. However, kinetic constraints of the two electrode (half cell) reactions, and the net resistive components of the cell, significantly reduce the energy conversion efficiency. As a result, the working voltage of the cell falls with increasing current drain. These losses are generally referred to as polarization loss and their minimization is a crucial objective in fuel cell research and development.
When a reagent is fed in solution to the electrode of an electrochemical cell, this is different from the situation in which a reagent like hydrogen is fed in gaseous form to the active electrode. This may be with respect to an anode of a fuel cell or of a redox flow cell or any other electrochemical cell. With the kinetic of mass transfer of the reacting species to the active reaction sites, the electrode becomes a major factor of overall polarization effects. This is in consideration of the diffusive mechanisms across the interface between the solid electrode and the solution and within the bulk of the solution, essentially governed by concentration gradients.
Under these conditions, a forced circulation of the solution of the reagent through the electrode compartment reduces polarization effects by equalizing concentration gradients in the bulk of the solution and preventing excessive localized depletion of the reagent species over the whole active area of the cell (electrode).
The advanced development of micro-machining technologies of monocrystalline silicon that has permitted the formation of microelectro-mechanical devices and sensors on silicon (MEMS) and the same technologies have also found application in the fabrication of electrochemical cells of micro to small size (up to wafer-size cells).
The relative small dimensions of silicon based electrochemical cells and batteries composed by a multicell stack and the relatively low mechanical strength of monocrystalline silicon bodies render the functional connection of the cell or stack of cells to a microfluidic system for the controlled feeding of a solution containing electrochemically reactive species to at least one of the electrodes of the cell problematic, and generally costly. This is because of the need to ensure an acceptable mechanical sturdiness of the connections and hydraulic sealing of the microfluidic feed system to be connected in a leak proof manner to the relative electrode compartment of a cell to the homologous or compartments of a multicell stack.